5/19/2008

The devastating earthquake in southwest China was so powerful that its seismic waves travelled around the globe - twice. Usually only surface waves from a quake of about magnitude 8.0 or stronger go around the globe more than once. The Matsushiro Seismological Observatory, north of Tokyo, detected surface quake waves at 3.41pm Japanese time (1641 AEST) on Monday, some 13 minutes after the 7.9-magnitude quake struck in China's Sichuan province. Seismological equipment in an underground tunnel showed the same kind of low-frequency waves 90 minutes later at 6.10pm and again at 8.40pm. This shows the waves from the quake went round the globe twice, travelling eastward from the epicentre to Japan, crossing the Pacific to the Americas and the Atlantic Ocean to Africa before coming back to Asia. Westbound waves were also observed. "The circling of waves was observed as the quake was so intense." Surface waves, which cannot be felt by humans, travel more slowly and last longer than waves that go through the interior of the Earth.